Everything moves slowly underwater because it's a thousand times denser than air and produces far more drag. But now researchers have found ways of overcoming this drag to create high-speed, and highly lethal, underwater weapons.

Standard torpedoes, like the American Mk 48, run at about thirty knots (50 kph).

The drag of the water seemed adequate until the 1980s when the Russians tested the K-222, a submarine which was clocked at over forty knots (74 mph).

In theory, such a submarine could simply turn around and outrun any torpedo fired at it.

The K-222 never made it beyond the prototype stage, but the threat was one of the reasons that the UK fielded the Spearfish torpedo.

This is the fastest in NATO; rather than a propeller, the Spearfish is driven by a gas turbine engine driving a pump jet, giving it a speed of 80 knots (148 kph) – "faster than a cheetah" as the makers say.

Going still faster requires a special type of technology known as supercavitation.

Normally, torpedoes are designed to be as streamlined as possible, but Russian researchers took the opposite approach with their Shkval (Squall) torpedo.

This has a blunt nose which pushes the water aside, creating a low-pressure zone around the torpedo.

At high enough speeds, the torpedo is surrounded by a bubble of water vapour, enclosing everything except the steering fins. The bubble is sustained by venting exhaust gases into it and it greatly reduces the friction on the torpedo, allowing it to reach an incredible 200 knots (370 kph). (end of excerpt)